This invention relates to a noninverting semiconductor memory cell, particularly suitable for making memory circuits having only one propagation delay from clock edge to output.
Random access memory (RAM) devices are becoming more widely used in the manufacture of digital equipment, particularly minicomputers, microprocessors and the like, as the speed and cost advantages of these devices increase. Static RAM cells and flip-flop circuits have been generally avoided, because of the delay in switching speed which results from the excessive space needed by the pair of cross-coupled inverters, or "latch", of each static RAM or flip-flop. This "pair of coupled inverters, is the basic structure of a most important and basic logic circuit, called a static latch". Herbert Taub, "Digital Circuits and Microprocessors," page 128 (McGraw-Hill Book Company, New York, 1982). Due to the coupled inverters, most circuits which use the "static latch" require at least two gate, i.e., propagation, delays from the clock edge to the output in order for the circuit to change logic states.
The history of integrated semiconductor circuit design has been characterized by a trend toward increasing speed and circuit densities. Various technologies have been invented to stimulate this trend. For example, transistor-transistor logic (TTL), was standard in digital equipment for a long time but has given way in many areas to N-channel metal oxide semiconductor (MOS) logic circuits because of their superiority in speed power product, packing density and ease of device fabrication. For these reasons, devices fabricated using these technologies are finding application primarily in memory, microprocessor, logic circuits, and the like. The Schottky barrier field effect transistor (MESFET) is another device that offers many of the advantages of the N-channel metal oxide semiconductor (MOS) technology without some of its disadvantages. Its primary application will also be in digital technology, such as memory circuits, random logic circuits, microprocessor type circuits and the like.
One of the major problems associated with MOSFET and MESFET technology, however, is the lack of noninverting logic circuits which provide only a one propagation delay switching speed. This is a very serious problem for it severely limits the number of complex logic gate types in which MESFET, and MOSFET, semiconductor devices can be used. For example, existing flip-flops are implemented using cross-coupled inverter logic gates. Due to the inverter cross-coupling, these flip-flops generally require two or three gate delays from the clock edge to the output in order for the device to change memory states.
Speed is one of the principal features of merit in any logic circuit. The term "speed" usually implies the speed at which the output changes from one state to another, i.e., the slope of the transition of the output; the delay in propagating a changed logic level through the circuit; and the rate at which the circuit can be cycled between the states, i.e., the repetition rate.
As a general principle, a semiconductor device, such as a transistor, a diode, and the like, will offer some delay in the speed at which the output signal of the device changes from one state to another in response to its input signal; i.e., the output signal will occur sometime after the input signal. Of course, every circuit, which comprises a number of circuit elements, including semiconductor devices, will therefore, also have some delay. This delay is generally known in the art as "propagation delay time", "delay time", or simply as "delay". Many circuit design failures, or design limitations, result from such undesired "delay"; for the speed at which an integrated circuit can perform a function is inversely proportional to the propagation delay of each circuit element, including the semiconductor devices in the integrated circuit. In other words, the shorter the propagation delay of the integrated circuit, the faster is its operational "speed".
In the world of digital technology, there is a very strong demand for faster and faster operational "speeds". Accordingly, considerable effort has been devoted toward the development of complex logic gates and memory circuits having minimal "delay".